1. Field
The present invention relates to an optical technique that offers, for example, forgery prevention effect, decorative effect and/or aesthetic effect.
2. Description of the Related Art
Securities, certificates, brands, media for personal authentication, etc. are required to be difficult to forge. Thus, in some cases, an optical device excellent in forgery prevention performance is provided on such articles.
Most of the optical devices include a microstructure such as diffraction grating, hologram, lens array, etc. The microstructures are hard to analyze. Further, in order to manufacture an optical device including the microstructure, an expensive manufacturing apparatus such as electron beam writer is necessary. For these reasons, the above optical devices achieve an excellent performance in forgery prevention.
Normally, the optical devices include a relief structure formation layer with a main surface having the microstructure and a reflective layer provided thereon. In this case, the reflective layer may be formed in a pattern on a part of the main surface in order to further enhance the forgery prevention effect. For example, when the reflective layer is provided on the main surface to have contours corresponding to micro-characters, a pattern that emits diffracted light and has a shape corresponding to the micro-characters can be obtained.
As the method of forming a patterned reflective layer, for example, photolithography can be used (for example, refer to Patent Document 1). This method allows forming a patterned reflective layer with high definition.
This method requires alignment of the relief structure formation layer with respect to a mask. Simultaneously achieving a high producibility and a high positional accuracy is, however, impossible or very difficult. For example, according to this method, a displacement of 100 μm or more may be produced between the target position and the contour of the reflective layer.
On the other hand, in Patent Document 2, employed are the following methods in order to form a reflective layer with a high positional accuracy.
In the first method, prepared first is a relief structure formation layer that includes a “first region” having a relief structure with a greater depth to-width ratio and a “second region” as a flat region or a region having a relief structure with a smaller depth-to-width ratio. Subsequently, a metal reflective layer is formed on the relief structure formation layer to have a uniform surface density. Then, the stacked body thus obtained is subjected to an etching treatment.
The portion of the metal reflective layer corresponding to the “first region” is lower in etching resistance than the portion corresponding to the “second region”. Therefore, the portion corresponding to the “first region” can be removed before the portion of the metal reflective layer corresponding to the “second region” is removed completely. That is, the metal reflective layer can be formed only on the “second region”.
According to this method, however, the portion of the metal reflective layer corresponding to the “second region” is partially removed by the etching treatment. For this reason, there is a possibility that the portion of the metal reflective layer corresponding to the “second region” has an excessively small thickness and thus has an insufficient reflectance. Alternatively, there is a possibility that the thickness of the metal reflective layer greatly varies at the portion corresponding to the “second region”. That is, according to this method, it is difficult to stably form the metal reflective layer.
In the second method, utilized is a difference between the transmittance of the portions of the above described stacked body corresponding to the “first region” and the “second region”. Specifically, utilized is the fact that the stacked body has a higher transmittance at the portion corresponding to the “first region” than at the portion corresponding to the “second region”.
That is, prepared first is a stacked body of a relief structure formation layer and a metal reflective layer. A photosensitive layer is formed on the metal reflective layer. Then, the stacked body is entirely irradiated with light from the relief structure formation layer's side. This makes it possible to cause a photoreaction at a higher efficiency in the portion of the photosensitive layer corresponding to the “first region”. Then, either one of the regions of the photosensitive layer corresponding to the “first region” and the “second region” is removed by treating it with a suitable solvent, etc.
Then, the metal reflective layer is subjected to an etching treatment using the partially removed photosensitive layer as a mask. Thus, either of the portions of the metal reflective layer corresponding to the “first region” and the “second region” is removed.
Since the difference between the above reflectances is generally small, the photoreaction also occurs in the portion of the photosensitive layer corresponding to the “second region”. Thus, it is in reality impossible or difficult to cause the above-described reaction in only one of the portions of the photosensitive layer corresponding to the “first region” and the “second region”. Therefore, when using this method, it is also in fact impossible or difficult to form a metal reflective layer with a high positional accuracy.
Further, this method requires an exposure process for the photosensitive layer. Therefore, this method is disadvantageous in cost and producibility.